Minor tweaks to terminal output, variables, and GRUB stanza

Minor tweaks to terminal output, variables, and GRUB stanza

* Print `Found grml-rescueboot ISO image:` instead of `Found Grml ISO
  image:` as the latter implies that the ISO image comes from Grml,
  whereas the former is more to the point of "grml-rescueboot found this
  image";

* Similarly, just print the ISO filename as the title, as "Grml Rescue
  System" implies that the ISO is the actual Grml rescue system;

* Use the existing `${indent}` variable instead of another eight spaces;

* Store `"${rel_dirname%/}/${grml}"` in a variable, as it is likely to
  be needed more than once;

* Add Debian bug reference for the `tpm` issue, as it is difficult for
  the user to understand otherwise;

* Use the existing GRUB variable to set the loopback path;

* Perform cleanup after exiting the ISO loopback menu.